The invention relates to a sensor system for the capacitive measurement of the fill level of a fluid medium and a medical device that includes such sensor system.
Sensors are required in a variety of applications in order to measure the fill level of fluids, especially electrically conductive fluids. When the requirements for the purity of the fluid and the prevention of contamination are high, and the fluid must not come into contact with the measurement apparatus, non-invasive measurement systems are suitable, which, for example, detect the fill level using an electrical field and the influencing of this field by the medium to be measured.
A capacitive fill level sensor is known from DE19949985A1, in which a first measurement electrode is attached to a side wall of a container and supplied via an amplifier with a voltage of a particular frequency, so that the electric field lines form in the manner of a capacitor to a second electrode that is disposed below the container. The measurement result may be distorted if, for instance, there is an increase in the container capacitance C (e.g. as a result of the temperature-dependence of the relative dielectric constant ∈R). For this reason a compensation electrode C is disposed on the wall of the container such that the field lines from this electrode run essentially through the container walls alone and thereby detect their influence.
As well as this influencing of the detected fill level by, for example, the changing properties of the container wall, the main challenge for the measuring apparatus is posed by the conductivity of the medium to be measured in combination with the susceptibility to wetting of the surface of the container, and the associated film formation on the interior side of the container when the fluid level falls. Possible surge-like variations in the fluid level must also be taken into account. Coupling also poses a problem. Coupling is the indicator of the quality of the accommodation of the container in the measurement apparatus. At the transitional point from the container to the receptacle, coupling capacitances CK arise. The impedance ZC of this transition is essentially expressed by the capacitive reactance XC=1/ωC=½πfCK.
The measurement path between the capacitor plates of the measurement apparatus is seen as a series connection of parallel-plate capacitors, with the capacitance of a parallel-plate capacitor calculated as: C=∈0*∈r*A/d.
∈0 is the dielectric constant (vacuum permittivity), A is the effective area, and d is the separation between the plates, which corresponds to the path of the electric field lines. ∈R is the relative dielectric constant, or relative permittivity, of the medium. In a relevant measurement medium, such as blood, isotonic saline solutions or similar, the relative dielectric constant ∈R is strongly frequency-dependent. On the other hand, the specific conductivity κ of the relevant media, at κ=6 (blood) or κ=16 (NaCl) mS/cm, is dependent on frequency only to a limited extent.
In the measurement apparatus, which will be described later in detail, the coupling capacitances vary in the pF region. The ohmic resistance of the medium is in the single-figure kΩ region. From these values it is clear that in the case of capacitive sensors the measurement result with an operating frequency f in the kHz region up to the single-figure MHz region is mainly determined by the coupling capacitances, since the impedance and phasing of the design is dominated by its reactance.
Known measurement apparatuses are unable to discriminate reliably between a true fill level and merely a thin film or surge of fluid, since the influence, which is only slight, of the difference in ohmic resistance of a massive medium from that of a thin film of fluid is outweighed by the smallest change in the coupling.
The necessity thus results of a high operating frequency, such as for example greater than 75 MHz, as explained in DE 19651355 A1 or DE 10 2005 057 558. On the other hand, high operating frequencies, i.e. for example 80 MHz or frequencies in the three-figure MHz region, place high demands on the design of the equipment and circuitry, and in particular the EMC compliance of the design.